An electrostatic recording film formed by laminating an insulating film with an electroconductive layer and a dielectric layer, respectively, is known.
In general, in an electrostatic recording system used in an electrostatic recording plotter for CAD, a recording voltage is applied to a multi-pin electrode head (hereinafter referred to as "pin electrode") to cause an aerial discharge in the narrow space (hereinafter referred to as "gap") between the pin electrode and the dielectric layer of the electrostatic recording film thereby to form an electrostatic latent image on the surface of the dielectric layer, and thereafter the latent image thus formed is developed with a liquid toner to give a visible image and this is then fixed as it is. In order to reproducibly obtain an image by the process, it is necessary that the gap is controlled to be in a predetermined range from Paschen's curve. For this, for example, a system where insulating grains are added so as to make the surface of the dielectric layer suitably rough, and the layer is brought into contact with the pin electrode, the surface roughness properly controlling the gap, is employed most generally. On the other hand, the fixation of the toner image on the electrostatic recording materials is effected by allowing (or drying) the image to stand as it is without the aid of heat or pressure in the conventional process. Therefore, in the case of using a paper support, a liquid toner permeates into the support, and whereby good fixability is obtained by merely air-drying, whereas in the case of using a polymer film support, since such permeation of a liquid toner into the support is hardly caused, sufficient fixability could not be obtained. Accordingly, there is a drawback that the toner image is easily peeled off from the dielectric layer when touched or rubbed with hands.
The electrostatic recording film is characterized by the desirable characteristics of recording stability, transparency, dimensional stability, strength, dust-repelling property and storage stability, as compared with the conventional electrostatic recording papers, and therefore the development of the film is desired. However, a recording film that gives a high quality image reliably in rapid recording and additionally has a sufficient image fixability is hardly obtained, which has heretofore been a great bar to the practical use of the film.
As a means of improving the fixability of the toner image, a recently published patent application mentions that a mixture of (A) at least one polymer selected from halogenated polyolefins containing from 60 to 80% by weight of halogen and (B) at least one polymer selected from polyester copolymers having Tg of 20.degree. C. or lower and/or poly(meth)acrylate copolymers having Tg of 20.degree. C. or lower are added to the dielectric layer as a combination of high polymer binders, the weight ratio of (A)/(B) being from 98/2 to 50/50 (JP-A-63-184758) (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application").
In accordance with the method, the fixability against release (as measured by applying and then peeling off a strip of mending tape) could surely be improved, but the fixability against scratch, e.g., with nails, could not be further improved. Accordingly, the film obtainable by the method could not be employed for drawings for CAD which are desired to be accurate.
The present inventors earnestly investigated and studied the problem of improvement of the fixability of the toner image to be formed on an electrostatic recording film with a liquid toner and, as a result, they have found a combination of high polymer binders which is effective for improving not only the fixability against release but also the fixability against scratching and accordingly they have achieved the present invention.